Heavy duty truck engine cooling fans typically sense the need for cooling fan operation by sensing engine coolant temperature, intake manifold air temperature and the pressure of refrigerant between the refrigerant compressor and condenser of the air conditioning system (often referred to as high-side pressure). Such fans are typically located between the truck engine and radiator, with the air conditioning system condenser and intake manifold port each disposed in the vicinity thereof. Engine cooling fans of this type are typically driven either in accordance with engine RPM, as disclosed in U.S. Pat. No. 3,853,098 to Ishikawa et al. and U.S. Pat. No. 3,894,521 to Sakasegawa et al., or by an independent electric motor as disclosed in U.S. Pat. No. 4,875,521 to Clemente and U.S. Pat. No. 4,881,494 to Ishigami.
It is known that the refrigerant high-side pressure in heavy duty vehicle air conditioning systems may become excessive under stopped and low vehicle speed conditions due to inadequate ram air flow across the condenser. Engine cooling fan operation is then required to reduce the high-side pressure to an acceptable level.
Engine cooling fan operation is typically controlled in response to high-side refrigerant pressure. Specifically, the engine cooling fan is activated when the high-side refrigerant pressure increases to a first predefined pressure range, due to inadequate air flow to the condenser. When the air flow to the condenser is increased under the influence of the engine cooling fan, the high-side pressure correspondingly decreases until it reaches a second lower predetermined pressure range, at which point the engine cooling fan is deactivated.
Under low vehicle speeds, it is known that high-side pressure increases rapidly due to inadequate ram air flow across the condenser, and decreases rapidly under the influence of the engine cooling fan. Under normal air conditioning system control, this rapid fluctuation in high-side pressure causes a corresponding rapid opening and closing of a pressure switch which, in turn, causes the engine cooling fan to correspondingly cycle on and off rapidly. Such fan cycling has the tendency to cause excessive fan clutch and fan belt wear. In addition, engine fan cycling generally causes excessive noise due to fan engagement and disengagement (attributable to fan belt and/or fan clutch squeal and other factors). Such fan engagement/disengagement is more noticeable in a heavy duty truck under stopped and idling conditions, which can be very disturbing to a heavy truck operator attempting to sleep in the cab with the air conditioning in operation.
Heretofore, it has been known to reduce engine fan cycling by utilizing a timer to maintain the fan, once activated due to excessive high-side pressure, in an activated state for a fixed time period. However, such fan operation is not needed after the vehicle has accelerated to a speed sufficient to provide adequate ram air flow past the condenser. Under such conditions, timed fan operation has the disadvantage of unnecessarily drawing power from the engine and thereby reducing fuel economy.
Another prior art approach to reducing engine fan cycling has been to provide constant fan operation, activated due to excessive refrigerant pressure, below a specified vehicle speed (typically below that which provides sufficient ram air flow). Examples of such systems are disclosed in U.S. Pat. No. 4,930,320 to Ide et al., U.S. Pat. No. 5,133,302 to Yamada et al., U.S. Pat. No. 5,285,650 to Lin and U.S. Pat. No. 5,307,644 to Cummins et al. However, such fan operation has the disadvantage of constantly drawing power from the engine at vehicle speeds wherein at least some ram air flow is provided to the condenser, so that full-time fan operation may not be necessary. Such excessive fan operation may result in reduced fuel economy.
What is therefore needed is an engine cooling fan control system that addresses the shortcomings of the prior art. Such a system should be reliable, inexpensive to implement, and readily integratable into an existing vehicle air conditioning system.